1. Field of the Invention
The present invention relates to a codec circuit having a programmable digital bandpass filter for matching the filter characteristics of the codec circuit to a transmitted PCM signal.
2. Description of Related Art
U.S. Pat. No. 5,212,817 discloses a programmable digital bandpass filter for a codec circuit for adaptation of the filter characteristics, in which case the setting filter coefficients can be set by changing the bandpass filter characteristic of the programmable digital bandpass filter.
A codec circuit is a circuit which intrinsically combines the functions of a coding switching device and a decoding switching device. When transmitting information, information is in many cases transmitted in both directions between two connections. In this situation, the codec circuit would carry out PCM (Pulse Code Modulation) signal coding in the transmission direction and PCM signal decoding in the reception direction.
In practice, codec circuits are normally in the form of digital signal processors (DSP), which are highly specialized, but nevertheless standardized, integrated circuits produced in large quantities for high-speed processing of a narrowly constrained set of input signals in real time. Digital signal processors such as these are typically used for data transmission by means of modems. A modem is a terminal which modulates outgoing signals and demodulates received signals.
For signal transmission of voice signals, the filter characteristics of the codec circuit have to comply with specific, predetermined specifications. There are no such specifications for the filter characteristics for signal transmission of modem signals.
Since the transmission rate of modems is directly proportional to the transmission frequency bandwidth, the data transmission rate increases with increasing frequency bandwidth of the digital filters integrated in the codec circuit. The digital filters which have been used in known codec circuits until now have filter characteristics matched to the specifications which exist for transmission of voice signals, and are not programmable. It is thus impossible when using such digital filters for codec circuits according to the prior art to widen the filter bandwidth, or to reset it, for data transmission when modem signals are being transmitted from a first modem to a second modem instead of the normal voice signals which originate, for example, from a telephone.